Niacin (vitamin B3) deficiency

Niacin (vitamin B3) deficiency is a clinically heterogeneous nutritional disorder that can affect the skin, gastrointestinal tract, and central and peripheral nervous systems. Severe deficiency causes pellagra, classically characterised by dermatitis, diarrhoea, and neuropsychiatric manifestations.

It can present with psychiatric features such as irritability, apathy, anxiety, insomnia, depression, behavioural change, and impaired concentration. Less commonly, psychotic symptoms such as hallucinations and delusions have been reported, including as prominent features of pellagra and in presentations initially diagnosed as a primary psychotic disorder.

Other characteristic features include cognitive impairment and encephalopathy, which may progress to confusion, delirium and, in severe cases, stupor or coma. Neurological manifestations can also include headache, tremor, paraesthesia, peripheral neuropathy, muscle weakness, ataxia and, less commonly, spasticity. The classical triad of dermatitis, diarrhoea and dementia is not present in all cases.

From a multisystemic lens perspective, clinical features in other body systems include (but are not limited to):

• Gastrointestinal: Diarrhoea, abdominal discomfort or pain, anorexia, nausea and vomiting, as well as glossitis, stomatitis and other oral mucosal changes.

• Dermatologic: A characteristic photosensitive dermatitis affecting sun-exposed areas, initially resembling sunburn and later becoming erythematous, scaly, hyperpigmented, thickened and rough. A sharply demarcated hyperpigmented band around the neck (“Casal necklace”) and glove- or boot-like distributions on the hands or feet are particularly distinctive.

• Musculoskeletal: Muscle weakness, which may accompany peripheral neuropathy, particularly in more advanced deficiency.

Early diagnosis is important because severe niacin deficiency can cause progressive neuropsychiatric and systemic complications and may be life-threatening if untreated. Prompt niacin replacement can result in substantial or complete recovery, whereas prolonged deficiency may lead to persistent neurological impairment. Because pellagra may present without the complete classical triad, a high index of suspicion is important, particularly in people with risk factors such as malnutrition, chronic alcohol use, malabsorption, restrictive diets, gastrointestinal disease or surgery, or medications associated with pellagra, particularly isoniazid and, to some extent, other agents such as phenytoin and carbamazepine.